Lactam dehydration method

ABSTRACT

The present invention relates to a process for separating the water from an aqueous lactam solution. 
     It applies more particularly to a mixture resulting from the reaction between an aminonitrile and water (this reaction also being called cyclizing hydrolysis). 
     The subject of the invention is a process for separating the water from an aqueous lactam solution, which process is economical both from the standpoint of the investment necessary for its implementation and from the standpoint of the operating cost.

The present invention relates to a process for separating the water froman aqueous lactam solution.

It applies more particularly to a mixture resulting from the reactionbetween an aminonitrile and water (this reaction also being calledcyclizing hydrolysis).

During the cyclizing hydrolysis of an aminonitrile in order to form alactam, one molecule of ammonia per molecule of lactam is also formed.At the end of the reaction, the reaction mixture therefore contains atleast the lactam produced, the excess water and ammonia.

It may contain lesser amounts of aminonitrile that has not reacted, orpossible by-products of the reaction.

It may also comprise a solvent possibly used in the cyclizing hydrolysisreaction.

The cyclizing hydrolysis may be carried out in vapour phase or in liquidphase. The process of the invention may apply to mixtures resulting fromone or other of these methods of preparation.

For the description of the various processes for preparing a lactam bycyclizing hydrolysis of an aminonitrile, reference may be made, forexample, to Patent EP-A-0,659,741 or to International Patent ApplicationWO-A-96/22974 or to International Patent Applications WO-A-95/14664 and95/14665.

The aminonitriles used for preparing lactams are more particularlyaminonitriles having from 4 to 12 carbon atoms, preferably linear orbranched aliphatic aminonitriles.

As examples of such aminonitriles, mention may be made of those whichderive from the hydrogenation of one of the two nitrile functionalgroups of dinitriles, such as adiponitrile, methylglutaronitrile,ethylsuccinonitrile, dimethylsuccinonitrile, succinonitrile,glutaronitrile and dodecanedinitrile, into a primary amine functionalgroup.

Among the solutions resulting from the cyclizing hydrolysis of anaminonitrile into a lactam, those which correspond to the preparation ofcaprolactam from 6-aminocapronitrile and water are the most importantfrom an industrial standpoint, since the said caprolactam, uponpolymerization, leads to nylon-6.

The process of the invention will therefore relate more particularly tothe distillation of the water from aqueous solutions of caprolactam, butthis distillation may also be transposed to aqueous solutions of otherlactams.

It is necessary beforehand to have separated, generally by distillation,the ammonia formed in the reaction.

One method of separating the water that may be envisaged consists indistilling the water at reduced pressure (less than or equal to 20 mbarabsolute, for example) so as not to exceed 145° C. at the bottom of thecolumn.

Such a process involves condensing the water thus distilled, bycirculating a fluid (such as water) at a temperature of less than orequal to 15° C. By way of example, this requires the presence of arefrigeration unit having a power of approximately 1.2 megawatts inorder to treat the order of 8 tonnes/hour of an aqueous lactam solutionhaving a solutes concentration of approximately 60% by weight.

The subject of the present invention is a process for separating thewater (which process may also be called dehydration) from an aqueouslactam solution, which process is more economical both from thestandpoint of the investment necessary for its implementation and fromthe standpoint of the energy consumed.

More specifically, it consists of a process for separating the waterfrom an aqueous lactam solution, characterized in that:

the water is distilled in a first distillation column by maintaining atemperature at the bottom of the column less than or equal to 160° C.and preferably less than or equal to 145° C., and at an absolutepressure of 50 millibars to 200 millibars, the distillate beingcondensed at a temperature of 30° C. to 60° C. using a coolant system;

the lactam remaining at the bottom of the column is then subjected to adistillation in a second distillation column by maintaining atemperature at the bottom of the column less than or equal to 160° C.and preferably less than or equal to 145° C., and at a pressure of 10 to45 millibars, so as to separate the water that it still contains;

the water, containing traces of lactam, distilled in the second column,is recovered in the form of vapour at a temperature greater than orequal to 70° C.

By operating the process according to the invention, it is not necessaryto have a refrigeration unit for condensing the water at the top of thefirst column.

The water, containing traces of lactam, distilled in the second columnis preferably recycled into the first distillation column. Thisrecycling may be carried out after condensing the water at the pressureof the second column, either between 10 and 45 millibars absolute,corresponding to a temperature of 5° C. to 30° C. However, such avariant only requires a refrigeration unit having a power ofapproximately 100 to 200 times less than that of the refrigeration unitwhich would have been necessary in the context of a distillation using asingle column.

According to another embodiment of the invention, the water containingtraces of lactam, distilled in the second column may be compressed bymeans of a steam ejector to a pressure allowing it to condense at atemperature greater than 15° C., using a standard coolant such as theatmospheric air or water at room temperature. In this case, the deliverypressure of the steam ejectors must be at least equal to the pressure ofthe first column, advantageously between 25 millibars absolute and 50 to200 millibars absolute, the pressure of the first column. In thisembodiment, the refrigeration units are omitted.

Overall, the present process is characterized, for the same capacity ofdistillation of an aqueous lactam solution, by a much lower total energyconsumption which can be broken down into an electricity consumptionwhich is 100 to 200 times less, as indicated previously, and anadditional energy consumption in the form of steam, in order to heat thesecond column, which represents only less than approximately 15% of theelectrical energy saving made.

The water recovered from the top of the first column is condensed at thepressure of this column, i.e. between 50 millibars and 200 millibarscorresponding to a temperature of between 35 and 60° C., compatible withmost coolants generally used, such as atmospheric air or river water orwater from cooling towers.

The water recovered from the top of the second column may be partiallycondensed at approximately 70° C. before being completely condensed andthen returned to the first column. This arrangement makes it possible tolimit the amount of water to be condensed under particular conditionsand therefore to reduce the amount of energy necessary for carrying outthis operation.

Before this condensed water is recycled into the first column, it may beadvantageous to add a certain amount of water in order to reduce thecaprolactam concentration and to prevent it from precipitating.

Furthermore, despite the need for an additional column, the investmentnecessary, for the same production, is also less in the processaccording to the invention than in a process carried out using a singlecolumn, on account of the considerable refrigerating capacity needed inthis latter process.

The aqueous lactam solution, preferably caprolactam, from which theammonia has been removed, has a lactam concentration which varies widelydepending on the conditions under which the said lactam is prepared,especially the initial water/aminonitrile molar ratio and whether or notan organic solvent is present. The lactam concentration may generallyvary from 5% to 80% by weight with respect to the total weight of thesolution, and preferably from 20% to 75%.

Since the water to be distilled has physical properties that are verydifferent from the lactam from which it must be separated, it is notnecessary to use distillation columns having a very large number oftheoretical trays.

Columns, particularly packed columns, with at least two theoreticaltrays are very suitable for implementing the process of the invention.Columns having a number of theoretical trays ranging from 2 to 10 may,for example, be used. The packing used may be of the loose packing orordered packing type, as proposed by the various manufacturers and sizedaccording to the rules of the art. It is also possible to use columnswith trays of the same efficiency, although this is less advantageousfrom the standpoint of head losses.

The process of the invention does not exclude the possibility ofseparating the water from the aqueous lactam solution using more thantwo columns.

However, in general this is unnecessary and, furthermore, the economicadvantages provided by the process described above would be fewer if thenumber of distillation columns were to be increased significantly. Theexamples which follow illustrate the invention.

EXAMPLE 1 Distillation with Two Columns in Series Operating at DifferentPressures

Distillation of an aqueous caprolactam solution having the followingcomposition:

water: 40% by weight

caprolactam: 60% by weight.

The characteristics of the first distillation column are as follows:

number of distillation stages (theoretical trays): 4

reflux ratio: 0.1

absolute operating pressure at the top of the column: 0.13 bar

top temperature: 50° C.

bottom temperature: 135° C.

heat load of the boiler: 3.9 Gcal/h

heat load of the condenser: 3.6 Gcal/h.

The characteristics of the second distillation column are as follows:

number of distillation stages (theoretical trays): 3

absolute operating pressure at the top of the column: 0.025 bar

top temperature: 130° C.

bottom temperature: 145° C.

heat load of the boiler: 0.08 Gcal/h

heat load of the 70° C. -moderated partial condenser: 0.02 Gcal/h

heat load of the condenser for the water to be recycled into the firstcolumn: 0.02 Gcal/h

power consumed by the refrigeration unit in order to condense the waterat the top of the second column: 10 kW, with condensation at 35° C.

The caprolactam, stripped of its water, which was obtained in thisexample has the following composition:

water: 0.010% by weight

caprolactam: 99.990% by weight.

Comparative Test Conventional Distillation Using a Refrigeration Unit atthe Top of the Column

Distillation of a product having the following composition:

water: 40% by weight

caprolactam: 60% by weight.

The characteristics of the distillation column are as follows:

number of distillation stages: 4

reflux ratio: 0.1

absolute operating pressure at the top of the column: 0.015 bar

top temperature: 13° C.

bottom temperature: 140° C.

heat load of the boiler: 3.6 Gcal/h

heat load of the condenser: 3.4 Gcal/h

power consumed by the refrigeration unit: 1200 kW, with condensation at35° C.

The caprolactam, stripped of its water, which was obtained in thecomparative test has the following composition:

water: 0.004% by weight

caprolactam: 99.996% by weight.

What is claimed is:
 1. A process for separating water from an aqueouslactam solution, consisting essentially of: distilling the aqueouslactam solution in a first distillation column while maintaining atemperature at the bottom of the column less than or equal to 160° C.and at an absolute pressure of 50 millibars to 200 millibars, distillaterecovered from the top of the column having been condensed at atemperature of 30° C. to 60° C. using a coolant; forwarding the lactamsolution remaining at the bottom of the first distillation column to asecond distillation column and maintaining a temperature at the bottomof the second distillation column less than or equal to 160° C. and at apressure of 10 millibars to 45 millibars; recovering water, containingtraces of lactam, at the top of the second column, as vapor at atemperature greater than or equal to 70° C.
 2. The process according toclaim 1, wherein the temperature maintained at the bottom of the firstcolumn is less than or equal to 145° C.
 3. The process according toclaim 1, wherein the temperature maintained at the bottom of the secondcolumn is less than or equal to 145° C.
 4. The process according toclaim 1, wherein the aqueous lactam solution is obtained by thecyclizing hydrolysis of 6-aminocapronitrile and the lactam iscaprolactam.
 5. The process according to claim 1, wherein the watervapor containing traces of lactam, recovered in the second column, isrecycled into the first column.
 6. The process according to claim 5,wherein the water vapor is recycled after condensing it at a temperatureof 5° C. to 30° C. at a pressure of 10 to 45 millibars.
 7. The processaccording to claim 5, wherein the water containing traces of lactams isrecycled after compressing the distilled water at a pressure at mostequal to the pressure of the first column and condensing at atemperature greater than 15° C.
 8. The process according to claim 5,wherein water is added to the water containing traces of lactam, whichis recovered in the second column, before it is recycled into the firstcolumn.
 9. The process according claim 1, wherein the first distillationcolumn and the second distillation column each have at least twotheoretical trays.
 10. The process according to claim 1, wherein thedistillation columns are packed columns.